The present invention relates to a one-dimensional image sensor for converting reflection light received from an original document irradiated by light into an electrical signal, which is applied to image reading devices such as FAXs.
There is shown in FIG. 11 a circuit diagram of a contact type one-dimensional image sensor used for conventional FAX reading devices. As shown in FIG. 11, it is the type of device in which outputs of a photo-transistor PTR are input to switches S1 to Sn to turn on in order, and read out on a common signal line 15.
The contact type photo-transistor one-dimensional image sensor as above is described in Japanese Patent Laying-open No. S61-124171.
However, in the one-dimensional image sensor as this there has been a problem that, because remaining carriers on a base of a photo-transistor PTR are removed through an emitter, the base potential cannot be reset to an initial state resulting in significant after-image.
Also, in order to obtain effective on-light outputs, before reading an original, the light source is turned OFF into a dark state to read out an off-light output and record its value, and thereafter the light source is brought into an ON state and light is irradiated onto the original and an on-light output is read out so that a difference from the formerly recorded off-light output is determined to thereby obtain an effective on-light output. However, this method has to have a memory to record the off-light output thus raising a problem of increased cost. Also, there are often cases that the aforesaid off-light output is not repeatedly set but is incorporated in the final product such as facsimiles wherein it is set only once at a shipping stage from the shop. When the off-light output has a temperature characteristic, there is a difference between the off-light output recorded at the shop before shipping and the off-light output upon actually reading the original, leading to a problem that an effective on-light output is not correctly determined with degradation in reproduced images.
Therefore, in order to solve such a conventional problem, it is an object of the present invention to supply with low cost an image sensor with reduced after-image and to determine an effective on-light output with accuracy.
In order to solve the above problem, the present invention structured an image sensor as follows.
(1) In an image sensor structured by a plurality of linear image sensor ICs in a straight line form, the image sensor ICs are structured such that the outputs of a plurality of photo-diodes are read out as sequential signal outputs through amplifiers connected to each photo-diode through a common signal line.
(2) The image sensor ICs are structured such that the output of each photo-diode includes two states of before reset operation of each photo-diode and upon resetting of the photo-diodes.
(3) The image sensor ICs are structured such that the output of each photo-diode includes three states of before reset operation of each photo-diode, upon resetting of the photo-diodes, and after resetting of the photo-diodes.
(4) The image sensor ICs are structured such that a reset switch is provided to temporarily bring the common signal line close to an initial state potential.